By design, Mode S surveillance radar equipment uses global 24-bit airframe addresses to keep track of the aircraft it is tracking.

Are these addresses also used by higher levels of the ATC system, such as to fill in call signs, flight plans and so forth automatically when the plane first appears in a controller's domain? Or does that still depend on ATC assigning 12-bit squawk codes that pilots set manually?

Does the Mode S address appear in a flight plan when filed?


3 Answers 3


The Mode S address is not used for identification of aircraft in the higher levels of the ATC system such as the Flight Data Processing System.

Mostly the assigned 12 bit code (also known as Mode A code or squawk code) is used to correlate a radar track to a flight plan. The major exception is that in Europe, when the Mode A code is set to 1000 (octal notation), the Mode-S downlinked Aircraft Identity (ACID) is used for flight plan correlation. This is to free up Mode A codes. Usually planned flights are assigned discrete Mode A codes but using ACID / Flight ID correlation, multiple aircraft on a flight plan in the same area can use Mode A code 1000.

The downlinked ACID must be equal to the Flight ID used to file the flight plan.

The Mode S 24 bit address is only used in low level systems; in the Mode S radar, in multilateration systems, in ADS-B receivers and to some extend in tracking systems.

  • $\begingroup$ The FAA document I found (see other answer) seems to imply that the ACID is also used by Canadian and Australian ATC in similar ways. $\endgroup$ Nov 12, 2014 at 18:08
  • $\begingroup$ @HenningMakholm That's is correct. As far as I know the ACID is used in Canada and Australia only for correlating ADS-B tracks to flight plans, not for Mode-S radar tracks. I am not 100% sure on that though. $\endgroup$
    – DeltaLima
    Nov 12, 2014 at 19:40

(Self-answer after some additional research).

It looks like the 24-bit addresses are not intended for such identification. Instead the Mode S protocol contains a way for the ATC radar to direct a transponder (identified by a 24-bit address) to send its call sign (known as "aircraft identification" in ICAO Annex 10.IV) piggybacked on a normal mode S altitude/squawk reply, coded as up to 8 alphanumeric characters.

It stands to reason that this is what higher-level ATC functions are based on.

Here is one source saying this is the case. It also implies that (some) ATC facilities don't normally assign discrete squawk codes to flights with working Mode S transponders.

(It's not clear to me why the FAA document says the FLT ID is only 7 characters; the data format defined by Annex 10 has room for 8).


This depends on the specific Air Traffic Navigation Service (ATNS) provider providing services and the equipment the controllers are equipped with. US-based FAA controllers actually are mostly unaware of which types of surveillance data are currently contributing to the position the system is plotting aircraft on their displays or scopes.

While they have access to all elements of ADS-B data associated with an aircraft which has been received, the need to query any beyond what is included with a legacy flight strip would be the exception, not the rule. The FAA maintains the world's largest and most complex integrated air traffic surveillance system which offers multiple tiers of aircraft tracking capabilities. This network of air traffic surveillance data sources offer coverage within the vast majority of the US National Airspace System (NAS) designated as Controlled Airspace (Class E, D, C, B, & A).

The FAA does maintain a database of all US registered Mode-S 24-bit aircraft address codes, colloquially referred to as "Aircraft Hex Codes". As a function of the many Bilateral and Multilateral Aviation Safety Agreements entered into with a number of foreign nation states, the FAA also populates this database with the Mode-S Hex Code provided by these foreign Civil Aviation Authorities (CAAs) with regularly received updates.

Nearly all FAA ATC surveillance functions associated with an aircraft's Hex-Code during normal operations are automated and seamless to the controller's display or scope. These functions include the auto population of registration number, aircraft type, and last reported equipment codes, etc into the track data blocks of nonparticipating aircraft, among others.

Your question specifically asks about tracking aircraft movements using these Mode-S Hex Codes. As far as the US FAA's policy, the answer is an unquestioning YES! All air traffic track data, to include all elements of ADS-B data are recorded and retained for a range of defined duration as dictated by specific facility and role. If there is a request or need to retain any set or portion of FAA AT data beyond those disposal time-frames, this can be done indefinitely.

Every nation state CAA and ATNS provider would need to be queried individually to determine the extent of their use of collected Mode-S Hex Code data. The only specific detail I'm aware of is that the ATNS providers located in the UK (NATS) and some parts of the EU (EuroControl) generally seem to be more aware of the Hex-Codes associated with the aircraft under their control. In fact, they are known to recognize and take notice when an aircraft Hex-Code mismatch is operating within their specific sectors. An aircraft Hex-Code mismatch occurs when the aircraft's reported, filed, or communicated registration number can't be reconciled with the Mode-S Hex-Code being used and transmitted and/or the aircraft records database they maintain.

Probably more than you wanted but I'm not one for brevity!

  • $\begingroup$ 2 things: 1. Eurocontrol is NOT the ANSP for Europe. It’s the ANSP for upper airspace Netherlands, Belgium, Luxembourg. ANSPs of (for example) Germany are DFS or of Italy ENAV! 2. You are talking about „HEX Codes“ all the time. At least in Germany that’s no way to identify any aircraft. Every Mode S Transponder either has a given FLT ID which was entered during installation or a changeable FLT ID (eg airliners). The systems work with this FLT ID. If an aircraft is squawking 1000 we will obtain idenfication using this FLT ID and comparing it to the according FLT ID in the submitted flightplans. $\endgroup$
    – pcfreakxx
    Jun 16, 2020 at 8:28
  • $\begingroup$ Cont.: The „CSMM“ system you refer to in the last part of your answer also does NOT compare the sent HEX with the stored HEX - it compares the FLT ID sent by the transponder with the FLT ID filed in the FPL. Also, the consequences are certainly not „usually nothing“. If an aircraft with wrong FLT ID is detected in your sector you have to advise the pilot who shall correct it. If she fails to correct it ATC must not identify this aircraft using squawk 1000 and we have to assign a discrete code. Reason being a mismatch btw FPL FLT ID and Mode S FLT ID won‘t correlate the target to a FPL. $\endgroup$
    – pcfreakxx
    Jun 16, 2020 at 8:37
  • 1
    $\begingroup$ @pcfreakxx You're absolutely right in both instances. I read the FAA order a little too quickly. My comment about 'usually nothing' was meant in a regulatory enforcement way. But that's unclear. I'll go ahead and modify the answer, good catches. $\endgroup$
    – BigNutz
    Jun 16, 2020 at 15:26

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